1. Field of the Invention
The present invention relates to a memory access control device to be employed in a DRAM (Dynamic Random Access Memory) such as an SDRAM (Synchronous DRAM), DDR (Double Data Rate) -SDRAM and more particularly to the memory access control device capable of enhancing access performance by predicting whether or not a hit is found.
The present application claims priority of Japanese Patent Application No. 2003-084790 filed on Mar. 26, 2003, which is hereby incorporated by reference.
2. Description of the Related Art
A conventional memory access control method is either an open page policy in which, when access to a DRAM is completed, a routine is terminated without pre-charging an accessed bank, or a closed page policy in which a routine is terminated after the accessed bank has been pre-charged.
If a bank is in a state where the bank has been already pre-charged and closed, the same bank, when being accessed next time, can be accessed at a constant speed regardless of its address. However, where a bank has not been pre-charged but has been opened, if a same page as its bank has previously opened is to be accessed, since opening of the page is not required, the bank can be accessed speedily when compared with the bank that has been closed. However, when accessing a page that is different from a page whose bank has been opened, the bank will be accessed at a slower speed when compared to the closed bank because the accessed page must be opened after the bank has been pre-charged.
Finding data on a given page is called a “hit” and failure to find data on a given page is called a “miss”.
A memory control device employing a conventional open page policy is disclosed in which access efficiency is improved by changing the timing with which a bank is closed according to the number of hits to a bank (Japanese Patent Application Publication No. 2001-166985).
Both the closed page policy and open page policy have problems. The closed page policy heightens access performance when the probability of a hit is low, while the open page policy heightens access performance when a probability of the hit is high. However, neither policy will enable a system to reach its theoretical limit for access performance.
Further, in a memory control device employing an open page policy, wherein the time at which a bank is closed is changed according to the number of hits to the bank, if misses are found continuously in a short interval, the access efficiency of the device will not improve because the bank is always kept open after each access attempt.
Moreover, the conventional memory control device has another problem. That is, when the shortest time with which a bank is closed is set to be shorter than an interval for access to a memory, since the operation becomes the same as that of the closed page policy, if many hits are found, there is no improvement of access efficiency.